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As the glacier crumbles and calves where it meets the ocean, millions of tons of ice drop into the sea every year. Despite the glacier's rapid flow, this collapse means Jakobshavn is actually retreating into the Greenland Ice Sheet. In 2012 and 2013, the ice front retreated more than a kilometer (a half mile) farther inland than in previous summers, the study reports.

Jakobshavn Isbrae is the largest contributor to sea level rise from Greenland, raising sea level by about 1 millimeter (0.04 inches) between 2000 and 2010.

Joughin said the deep valley beneath the glacier is responsible for the sudden change in speed starting in 2012. Though Greenland's ice sheet was hit with a massive surface melt in August 2012, which may quicken sliding glaciers, the Jakobshavn Isbrae Glacier first reached the deeply carved valley in 2012.

In the near future, because of Jakobshavn's deep valley, the glacier could flow even faster, the researchers report. "The increase [in flow] likely could reach or exceed a factor of 10 within decades," they wrote.

However, the finish line is in sight -- within about a century. The deep valley ends after another 31 to 37 miles (50 to 60 km). Jakobshavn's retreat may slow once it hits bedrock, Joughin said.

A similar story could play out at other Greenland glaciers, researchers think. Recent studies suggest the shape of glacial valleys can influence how quickly glaciers retreat, and that some glaciers may slow when they shrink back far enough to hit narrow chokepoints.